Some conception may be had of the enormous vegetable deposits of the Carboniferous era by recalling the fact that the resultant coal in each seam is only from one ninth to one sixteenth in bulk of the woody fibre from which it has been derived, the loss being mainly in oxygen and hydrogen. It is probable that the coal seams as well as the rock strata had attained a comparative degree of hardness before the close of the Carboniferous age. It was at the close of this age that those profound disturbances of the earth’s crust throughout eastern North America took place which have already been referred to. Hitherto, through the long ages of Paleozoic time, there had been comparative quiet. As cooling and contraction of the earth’s body were still going on, there were doubtless oscillations of surface and subsidence of strata in almost continuous progress. But these movements were very slow, amounting, perhaps, to not more than a foot in a century. Yet in Pennsylvania and Virginia the sinking of the crust up to the close of the Carboniferous age amounted to 35,000 or 40,000 feet. That the subsidence was quiet and unmarked by violent movement is attested by the regularity of strata, especially of the carboniferous measures, which alone show a sinking of 3,000 or 4,000 feet. Neither were the disturbances which followed violent, nor were the changes paroxysmal. Indeed, the probability is that they took place gradually through long periods of time. They were, nevertheless, productive of enormous results in the shape of hills, peaks, and mountain ranges. These movements in the earth’s crust were due, as always, to contractions in the earth’s body or reductions in its bulk. On the same principle by which the skin of an apple that has dried without decay is thrown into folds and wrinkles, the earth’s crust became corrugated. There is this difference, however: the crust, being hard and unyielding, has often been torn and broken in the process of change. Naturally these ridges in the earth’s surface have been lifted along the lines of least resistance, and these lines seem to have been, at the time of the Appalachian revolution, practically parallel to the line of the Atlantic coast, though long spurs were thrown out in other directions, isolated dome-shaped elevations were raised up, and bowl-shaped valleys were hollowed out among the hills.